[27litres]

No, very little rust concerns here with modern cars.
Not to say that rust doesn't happen, but our cars don't tend to biodegrade to the extent of yours!

There are exceptions:
My missus' old Ford Falcon ("old" = 2003) is a wagon (estate), and when Ford used to make Falcon wagons, they stretched the wheelbase by 100mm or so for extra load carrying (and used the extended architecture for a long wheelbase "prestige" sedan called the Fairlane (a-la A8L - except the overall size was only 16mm longer than the S8!)).
They did it on the cheap (can't picture Ford being like that..) and the stretch was basically a rear wheelarch sheet metal extension (using the standard Falcon sedan rear door) which had the net effect of enlarging the inner guard area and created a cavity area between the door and wheelarch of the extra 100mm or so.
We live in a dirt road with a dirt driveway and are on the southern slope of the local hill (think northern exposure there). So during winter it remains somewhat soggy down here and mud sits in the cavity area and remains damp. If you're not diligent in hosing that area out frequently, it becomes a rust hotspot (as in my missus case!).